Collins Russian Dictionary 2nd Edition © HarperCollins Publishers 2000, 1997:
sixth [sɪksθ] adj шесто́й
n (fraction) одна́ шеста́я f adj, шеста́я часть f
the upper/lower sixth (BRIT) (SCOL) пе́рвая/втора́я ступе́нь выпускно́го кла́ссаsee also fifth
* is used to mark translations which have irregular inflections. The Russian-English side of the dictionary gives inflectional information.